Taken this is WinForms: For example with decimal values use
Decimal.Parse Method[
^]. For example:
textBox3.Text = (Decimal.Parse(textBox1.Text) * Decimal.Parse(textBox2.Text)).ToString();
Of course you need to add checks, use
try catch
, possibly use TryParse instead and so on.
Addition:
You can put the code where anywhere you like. For example you can use the text boxes Validating event, KeyUp, KeyDown etc, whatever is feasible.